If you were to run it now, I'd say no. The calculators are notoriously optimistic, though your half and 10K line up very well, and considering your age (older runners tend to have more endurance vs speed than younger runners) I'd say it's not far off, but still, optimistic. I think sub-4 will be a pretty good bet, though.

However, you still have a long way to go to your marathon, and as is evidenced by the discrepancy taylormade pointed out, you're improving rapidly (typical for new runners). Your half is over 3 months old and your 10K even older. Run another 10K or half in late September or early October, and then post here with the information in the sticky about not just your time but your training program, and we will be able to give you better advice on a realistic goal.

I think it's hard to gauge endurance without knowing miles per week and what kind of long runs you've done. Yes it is possible, but aggressive depending on your current fitness level. It's too early to worry about if you can pull it off. But I would say that you should keep that 3:50 in your sights as you run your race-pace miles in whatever plan you choose. Then you can tweak the numbers as you progress.